North Korea says it will deport an Australian missionary detained for spreading Christianity in the country, saying he apologised for his anti-state religious acts and requested forgiveness.

Authorities in North Korea have been investigating John Short since his arrest for secretly spreading Bible tracts near a Buddhist temple in Pyongyang on February 16, the birthday of late leader Kim Jong Il, the North's official Korean Central News Agency said.

The report said that Short, 75, admitted he committed a crime that hurt the Korean people's trust in their leaders and apologised for his behaviour.

''I now realise the seriousness of my insult to the Korean people on February 16th because I made the Korean people angry and for this I truly apologise,'' Short was quoted as saying in a written apology, according to separate KCNA report.

''I am willing to bow down on my knees to request this tolerance of (North Korea) and the Korean people.''

KCNA says North Korea decided to expel him thanks to the tolerance of the country's laws and in consideration of his age.

North Korea's constitution guarantees freedom of religion, but in practice only sanctioned services are tolerated by the government.

Defectors from the country have said that the distribution of Bibles and secret prayer services can mean banishment to a labour camp or execution.

North Korea typically frees foreign detainees after they've admitted their crimes, but many say after their release that their confessions were given involuntarily and under duress.

Last week, North Korea presented to the media a detained South Korean Baptist missionary who apologised for allegedly trying to reach Pyongyang with Bibles, Christian instructional materials and movies in October.

North Korea has been holding a Korean-American missionary, Kenneth Bae, since November 2012. Bae, sentenced to 15 years of hard labour for hostile acts, held a similar news conference to apologise for his behaviour.

KCNA on its website posted video showing a calm-looking Short, dressed in a black jacket, reading what appeared to be his written apology before taking a quick bow at a room.

Kim Jong Il's birthday and that of his father and North Korea founder Kim Il Sung were the nation's biggest holidays.

Kim Jong Il died in late 2011 and his son Kim Jong Un took over power.

Australia's Department of Foreign Affairs and Trade said it was trying to confirm North Korea's announcement on Short.